The fermentative production of succinic acid (SA) from biomass has already drawn much attention because said acid represents an important constituent of synthetic resins or is a source of further valuable low-molecular chemical compounds, in particular tetrahydrofuran (THF), 1,4-butanediol (BDO), gamma-butyrolactone (GBL) and pyrrolidones (WO-A-2006/066839).
A SA-producing bacterium isolated from bovine rumen was described by Lee et al (2002a). The bacterium is a non-motile, non-spore-forming, mesophilic and capnophilic gram-negative rod or coccobacillus. Phylogenetic analysis based on the 16S rRNA sequence and physiological analysis indicated that the strain belongs to genus Mannheimia as a novel species, and has been named Mannheimia succiniciproducens MBEL55E. Under 100% CO2 conditions, it grows well in the pH range of 6.0-7.5 and produces succinic acid, acetic acid and formic acid at a constant ratio of 2:1:1. When M. succiniciproducens MBEL55E was cultured anaerobically under CO2-saturation with glucose as carbon source, 19.8 g/L of glucose were consumed and 13.3 g/L of SA were produced in 7.5 h of incubation.
A significant drawback of said organism is, however, its inability to metabolize glycerol, which, as a constituent of triacyl glycerols (TAGs), becomes readily available e. g. as by-product in the transesterification reaction of Biodiesel production (Dharmadi et al., 2006).
The fermentative production of succinic acid from glycerol has been described in the scientific literature (Lee et al., 2001; Dharmadi et al., 2006) and with glycerol higher yields [mass of SA produced/mass of raw material consumed] than with common sugars like glucose were achieved (Lee et al., 2001). However, the space time yield obtained with glycerol was substantially lower than with glucose (0.14 vs. 1.0 g SA/[L h]) and no crude glycerol was used.
Only in a few cases anaerobic metabolisation of glycerol to fermentation products have been described. E. coli is able to ferment glycerol under very specific conditions such as acidic pH, avoiding accumulation of the fermentation gas hydrogen, and appropriate medium composition. (Dharmadi et al 2006, Yazdani and Gonzalez 2007) Many microorganisms are able to metabolize glycerol in the presence of external electron acceptors (respiratory metabolism), few are able to do so fermentatively (i.e. in the absence of electron acceptors). The fermentative metabolism of glycerol has been studied in great detail in several species of the Enterobacteriaceae family, such as Citrobacter freundii and Klebsiella pneumoniae. Dissimilation of glycerol in these organisms is strictly linked to their capacity to synthesize the highly reduced product 1,3-propanediol (1,3-PDO) (Dharmadi et al 2006). The conversion of glycerol into succinic acid using Anaerobiospirillum succiniciproducens has been reported (Lee et al. 2001). This study demonstrated that succinic acid could be produced with little formation of by-product acetic acid by using glycerol as a carbon source, thus facilitating purification of succinic acid. The highest yield was obtained by intermittently feeding glycerol and yeast extract, a strategy that resulted in the production of about 19 g/L of succinic acid. It was noted, however, that unidentified nutritional components present in yeast extract were needed for glycerol fermentation to take place.
Carboxylation reactions of oxaloacetate catalyzed by the enzymes phopshoenolpyruvate carboxylase (PEPC), phopshoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PycA) are utilizing HCO3− as a source of CO2 (Peters-Wendisch, P G et al). Therefore hydrogencarbonate sources such as NaHCO3, KHCO3, NH4HCO3 and so on can be applied to fermentation and cultivation media to improve the availability of HCO3− in the metabolisations of substrates to succinic acid. The production of succinic acid from glucose has not been found to be dependent on the addition of HCO3− in the prior art so far.
Biomass production by anaerobic organisms is limited by the amount of ATP produced from fermentative pathways. Biomass yield of glycerol in anaerobic organisms is lower than of saccharides, like hexoses such as glucose, fructose, pentoses such as xylose arabinose or disaccharides such as sucrose or maltose (Lee et al. 2001, Dharmadi 2007).
Saccharides, however, theoretically can be converted to succinic acid with a significantly lower yield than glycerol due to the lower reduction state of saccharides compared to the polyol glycerol. The combination of saccharides with glycerol have been found to function in an succinic acid producing anaerobic organisms (Lee et al. 2001), however without reaching succinic acid titers beyond 28 g/l.
There is, therefore, a need for further bacterial strains, which have the ability to produce organic acids, in particular SA, from glycerol. In particular, such strains should produce said acids with high productivity from glycerol, especially if crude glycerol e. g. from bio diesel production can be used without prior purification.